The fixed, portable and mobile radio telecommunications field is a highly developed technology that enjoys a wide variety of applications. Many service businesses such as taxi, police, firemen, rescue squads, mobile service and maintenance repairmen, etc. constitute applications well adapted for fixed, portable or mobile radio communications. Such type of communications are easily and readily established, simply by operating a push-to-talk switch on the radio, receiving a confirmation "beep", and beginning the voice communications. The voice message is automatically broadcast to all the radio receivers assigned to the same group. When the voice communication is completed, the speaker simply releases the push-to-talk switch, and others in the group can then respond in a similar manner. Although only one radio of the group can transmit at a particular time in the dispatch mode, the period of transmission is generally short, thereby allowing others in the group to respond by activating their push-to-talk switches and speak accordingly. In this manner, and in contrast to mobile cellular telephones, no dial-up number or other similar connection is required and thus communications are easily initiated without having to remember or dial telephone numbers or memorize the various sequences that may be involved.
The push-to-talk radio communications described above are commonly known as "dispatch" radio communications. In such type of communications, a communication cell or site includes transmit/receive equipment for broadcasting the voice messages to the other mobile radios served by the cell. Indeed, each cell can include many distinct groups of radios so that the users of a particular group can communicate therebetween, without interfering with the communications of the other groups. In such type of arrangements, the cell includes a controller and a number of repeaters, generally up to twenty repeaters, which are trunked to provide the dispatch communications between the various groups within the cell.
Some types of fixed, portable and mobile communication radios are also available which provide both dispatch-type of calls and full duplex calls. In a full duplex communication call, a push-to-talk switch is not utilized. Rather, a bi-directional telecommunication path is maintained between the originating radio or telephone and the destination radio or telephone during the duration of the communication. Although the full duplex type of communication path allows for a continuous bi-directional exchange of voice messages, such path is dedicated during the time of use, and cannot be used by other mobile radios or equipment.
Each communication radio cell includes a transmit and receive antenna for providing communication services to the radios within a geographical radius of about 20-40 miles, depending upon the transmitting power, the terrain, weather, etc. The radio communication technology has advanced such that it is now possible to network a number of cells together, using the public switched telephone network (PSTN). In such an arrangement, the voice messages are transferred by way of voice-grade telephone lines or digital carrier lines from one cell to the other. However, the set-up of the voice path from one cell to the other is initially established by a data line between the cells and a modem for communicating set-up data information. Such type of arrangement is also highly useful where "roaming" of a radio is allowed from one cell to another cell. With the roaming feature, when the radio is turned on, data is periodically transmitted from the radio to the nearby cell. When a cell detects the new appearance of a mobile radio, it broadcasts the location of the radio, via a data line, to the other cells of the network. In this manner, all cells of the network can update appropriate tables to maintain an account of the location of the various radios.
While the use of a full duplex radio communication link to network plural cells together is relatively easy, namely, maintaining the voice line off-hook during the entire bi-directional exchange of voice signals, the networking of a dispatch-type of calls between plural cells becomes much more difficult. Indeed, although it is not uncommon to network a first radio cell by way of telephone voice lines with a second radio cell for dispatch communications, the networking of more than two cells becomes very complicated. This is due primarily to the number of different push-to-talk conversations or messages that generally take place in order to complete the overall exchange of information. Moreover, the time delay in transferring the "key" commands and "dekey" commands (that initiate and terminate the push-to-talk message) becomes significant, and indeed increases as the number of networked cells increase. Moreover, an algorithm to resolve conflicts in simultaneous transmission attempts becomes much more complex as the number of networked cells increase.
From the foregoing, it can be seen that a need exists for a technique for networking plural cells together to easily and efficiently carry out dispatch-type of radio communications. A further need exists for a radio telecommunications system having a hub controller for controlling the usage of voice lines between cells of a network to coordinate dispatch-type messages. Another need exists for a method of communicating data commands between originating radio cells and destination radio cells to accommodate the communication of different groups of users serviced by the network.